1. Field of the Invention
This invention relates generally to implantable devices, such as stents. More particularly, the present invention relates to an apparatus and method for coating stents.
2. Description of the Background
Implanting a stent, after a percutaneous transluminal coronary angioplasty (PTCA) procedure, is often used to avoid or mitigate the effects of restenosis at a surgical site. In general, stents are small, cylindrical devices whose structure serves to create or maintain an unobstructed opening within a lumen. Stents are typically made of, for example, stainless steel, Nitinol or other materials and are delivered to the target site via a balloon catheter. Although the stents are effective in opening the stenotic lumen, the foreign material and structure of the stents themselves may exacerbate the occurrence of restenosis or thrombosis.
In addition to using a stent, drugs or therapeutic agents that limit migration and/or proliferation of vascular smooth muscle cells are used to significantly reduce the incidence of restenosis and thrombosis. Examples of therapeutic agents commonly used include heparin, aspirin, IIb/IIa inhibitors, antithrombogenic agents, dexamethasone, steroids, antiinflammatory agents, cytostatic agents, cytotoxic agents, antimicrobials, thrombolytics, monoclonal antibodies, tranilast, and antifibrosis agents. Since the therapeutic agents are applied systemically to the patient, they are absorbed not only by the tissues at the target site, but by other areas of the body. As such, one drawback associated with the systemic application of drugs is that areas of the body not needing treatment are also affected. To provide more site-specific treatment, stents are frequently used as a means of delivering drugs exclusively to the target site. Drugs are suspended in tissue-compatible polymers such as silicones, polyurethanes, polyvinyl alcohol, poly(ethylene-co-vinyl alcohol), polyethylene, hydrogels, substituted methacrylates, poly(ethylene-co-vinyl acetate), and hyaluronic acid and blended mixtures thereof. By positioning the stent at the target site, the drugs can be applied directly to the area of the lumen requiring therapy.
Although stents with a drug coating have been an advance for the treatment of restenosis and other similar vascular ailments, the stents, and the methods and apparatus for their production have not yet been perfected. For instance, conventional techniques often apply a single coating of a homogenous composition that contains a mixture of a polymer and a therapeutic substance. The use of a homogenous composition may have several flaws. The polymeric portion of the coating may not be stable in the vascular environment (i.e., the polymer may leach into the blood), and may not be capable of holding a sufficient amount of the drug. In addition, conventional coatings may not have a blood compatible surface to the vascular environment. Moreover, the drug release rate of a coating made from a homogenous composition cannot be tailored to provide for different release profiles.
As an alternative to using a homogenous composition to coat a stent, some conventional techniques apply a coating to a stent that has more than one layer, with each layer having a different composition. These techniques also suffer from some flaws. For example, the different layers may not strongly adhere to each, thereby allowing one or more layer to leach into the blood or become detached creating an embolization hazard. Also, the coating process of these techniques may not be very efficient because each layer must be applied, and then dried before the next layer is applied. Finally, the application of the composition for each additional layer subsequent to the drying of the previously applied layer can cause the extraction of the drug out of the previous layer. Accordingly, the concentration of the drug will reside in the upper most layers, causing a rapid release of the drug subsequent to the implantation procedure. This “burst-effect” leads to a reduced residence time of the drug at the implantation site, which may be undesirable depending on the type of condition being treated.
Accordingly, what is needed is an apparatus and process for coating stents that does not suffer from the aforementioned drawbacks. More particularly, there is a need for a method and apparatus for coating a stent that is able to modify the coating formulation as the formulation is being applied to the stent.